This article is from the Feline Leukemia (Cats) FAQ, posted to rec.pets.cats newsgroup. Maintained by Erin Miller with numerous contributions by others.
There is no sure way to keep your cat healthy. Eventually, an FeLV-related disease will probably develop no matter what you do. However, one way in which a disease is likely to develop is if you stress your cat's system. If a cat's system is stressed, its body can't put as much energy into fighting off illnesses (just like you always seem to get sick during exams or when you have to make a presentation to your boss). "When all my cat does is eat and sleep 20 hours a day, how can it possibly be stressed?" you may ask. Anything which is upsetting or unpleasant for your cat may stress it, such as going for very long periods without food or water, overcrowding, movement to new territory, territorial conflicts, sending your cat to be boarded for long periods of time, or pregnancy and lactation. (Of course one should NEVER breed a FeLV+ queen as it will expose not only the tom, but all the kittens will be FeLV+. There is also an indication, although no proof, that FeLV causes abortions in queens. For the same reasons, neither should FeLV+ toms be bred.) Basically, keep in mind things which you have noticed in the past that seriously upset your cat. These are things which are more likely to lower your cat's natural immune system and give a disease the chance to attack.